Random access channel preamble detection

ABSTRACT

A preamble for a random access channel (RACH) transmission is provided by selecting one out of sixteen preamble signatures. Each of the sixteen preamble signatures having sixteen symbols. A code is produced based on the preamble sequence. The produced code is phase rotated to produce a preamble code.

CROSS REFERENCE TO RELATED APPLICATION(S)

The application is a continuation of U.S. patent application Ser. No.09/868,209, filed Jun. 14, 2001, which is a 371 of PCT/US99/29504, filedDec. 14, 1999, which are incorporated by reference as if fully setforth.

FIELD OF INVENTION

The present invention relates generally to transmission systems andmethods for binary modulated signals. More specifically, the inventionrelates to a CDMA transmission system for transmitting a modulatedsignal in a mobile communications environment where transmitting rangevaries.

BACKGROUND

A communication system has one principle function, to transmitinformation from a source to a destination. The information generated bythe source is typically an electrical signal that changes with time

The information signal is transmitted from the source to the destinationover an appropriate medium, usually referred to as a channel. One methodof altering the information signal to match the characteristics of thechannel is referred to as modulation. The recovery of theinformation-bearing signal is called demodulation. The demodulationprocess converts the transmitted signal using the logical inverse of themodulation process. If the transmission channel were an ideal medium,the signal at the destination would be the same as at the source.However, the reality is that during the transmission process, the signalundergoes many transformations which induce distortion. A receiver atthe destination must recover the original information by removing allother effects.

Most communications currently rely upon the conversion of an analogsource into a digital domain for transmission and ultimatelyreconversion to analog form depending upon the type of informationconveyed. The simplest digital representation is where the informationin any bit time is a binary value, either a 1 or a 0. To extend thepossible range of values that the information can be, a symbol is usedto represent more than two possible values. Ternary and quaternarysymbols take on three and four values respectively. The varying valuesare represented by integers, positive and negative, and are usuallysymmetric. The concept of a symbol allows a greater degree ofinformation since the bit content of each symbol dictates a unique pulseshape. Depending upon the number of levels of a symbol, an equal numberof unique pulse or wave shapes exist. The information at the source isconverted into symbols which are modulated and transmitted through thechannel for demodulation at the destination.

The normal processes of a communication system affect the information ina calculable and controllable manner. However, during the transmissionfrom a source to a destination, a component that cannot be calculated isnoise. The addition of noise in a digital transmission corrupts thesignal and increases the probability of errors. Other signal corruptionsthat manifest themselves are multipath distortions due to naturalterrain and manmade structures, and distances the signals travel whichaffect signal timing. The communication system needs to define thepredictable transformations that the information signal encounters andduring reception of the information the receiver must possess the meansto analyze the predictable transformations that have occurred.

A simple binary transmission system could use a positive pulse for alogical 1 and a negative pulse for a logical 0, with rectangular pulseshapes transmitted by the source. The pulse shape received at thedestination undergoes the aforementioned transformations including noiseand other distortions.

To minimize the probability of error, the response of a filter used atthe receiver is matched to the transmitter pulse shape. One optimalreceiver, known as a matched filter, can easily determine whether atransmitted pulse shape is a logical 1 or 0 and is used extensively fordigital communications. Each matched filter is matched to a particularpulse shape generated by the transmitter corresponding to a symbol. Thematched filter is sampled at the symbol rate to produce an output thatcorrelates the input pulse shape with the response of the filter. If theinput is identical to the filter response, the output will produce alarge value representing the total energy of the signal pulse. Theoutput usually is a complex quantity that is relative to the input. Theoptimum performance of the matched filter depends on a precise replicaof the received signal pulses which requires accurate phasesynchronization. Phase synchronization can easily be maintained with theuse of a phase-locked loop (PLL). Pulse synchronization, however, is aproblem for matched filters. If the pulses are not time-aligned to onesymbol time, intersymbol interference (ISI) appears.

An example prior art communication system is shown in FIG. 1. The systememploys a technique known as code division multiplexing, or morecommonly, as code-division multiple access or CDMA.

CDMA is a communication technique in which data is transmitted within abroadened band (spread spectrum) by modulating the data to betransmitted with a pseudo-noise signal. The data signal to betransmitted may have a bandwidth of only a few thousand Hertzdistributed over a frequency band that may be several million Hertz. Thecommunication channel may be used simultaneously by m independentsubchannels. For each subchannel, all other subchannels appear as noise.

As shown, a single subchannel of a given bandwidth is mixed with aunique spreading code which repeats a predetermined pattern generated bya wide bandwidth, pseudo-noise (pn) sequence generator. These uniqueuser spreading codes are typically orthogonal to one another such thatthe cross-correlation between the spreading codes is approximately zero.A data signal is modulated with the pn sequence to produce a digitalspread spectrum signal. A carrier signal is then modulated with thedigital spread spectrum signal, to establish a forward-link, andtransmitted. A receiver demodulates the transmission and extracts thedigital spread spectrum signal. The transmitted data is reproduced aftercorrelation with the matching pn sequence. When the spreading codes areorthogonal to one another, the received signal can be correlated with aparticular user signal related to the particular spreading code suchthat only the desired user signal related to the particular spreadingcode is enhanced while the other signals for all other users are notenhanced. The same process is repeated to establish a reverse-link.

If a coherent modulation technique such as phase shift keying (PSK) isused for a plurality of subscriber units, whether stationary or mobile,a global pilot is continuously transmitted by the base station forsynchronizing with the subscriber units. The subscriber unitssynchronize with the base station at all times and use the pilot signalinformation to estimate channel phase and magnitude parameters.

For the reverse-link, a common pilot signal is not feasible. For initialacquisition by the base station to establish a reverse-link, asubscriber unit transmits a random access packet over a predeterminedrandom access channel (RACH). The random access packet serves twofunctions. The first function is for initial acquisition when thesubscriber unit is transmitting and the base station has to receive thetransmission quickly and determine what is received. The RACH initiatesthe reverse-link to the base station. The second use of random accesspackets is for communicating lower data rate information rather thanconsuming a dedicated continuous voice communication channel. Smallamounts of data such as credit card information are included in the dataportion of the random access packet instead of call placing data. Theinformation when sent to the base station can be forwarded to anothercommunicating user. By using the random packet data portion to transportaddressing and data, available air resources are not burdened and can beefficiently used for higher data rate communications.

A random access packet comprises a preamble portion and a data portion.The data may be transmitted in parallel with the preamble. In the priorart, the random access channel typically uses quadrature phase shiftkeying (QPSK) for the preamble and data.

The base station examines the received preamble for the unique spreadingcodes. Each symbol of the RACH preamble is spread with a pn sequence.Using matched filters, the base station searches continuously for thosecodes that correlate. The data portion contains instructions for adesired service. The base station demodulates the data portion todetermine what type of service is requested such as a voice call, fax,etc. The base station then proceeds by allocating a specificcommunication channel for the subscriber unit to use for thereverse-link and identifying the spreading codes for that channel. Oncethe communication channel is assigned, the RACH is released for othersubscriber units to use. Additional RACHs afford quicker base stationacquisition by eliminating possible collisions between subscriber unitssimultaneously initiating calls.

Without a subscriber unit pilot signal providing pulse synchronizationin the reverse-link, acquisition of the RACH from a mobile subscriberunit is difficult if a coherent coding technique such as PSK is usedcompounded with transmitting range ambiguity. Since a mobile subscriberunit is synchronized with the base station, the RACH preamble istransmitted at a predefined rate.

An example prior art preamble signature is defined by 16 symbols. Atable of sixteen coherent RACH preamble signatures is shown in FIG. 2.Since each symbol is a complex quantity and has a pulse shape comprising256 chips of the spreading pn sequence, each signature comprises 4096chips. The complete RACH preamble signature is transmitted at a chippingrate of 4096 chips per millisecond or 0.244 chips per microsecond.

From the global pilot signal, each subscriber unit receives frameboundary information. Depending upon the distance from the base stationto a subscriber unit, the frame boundary information suffers aforward-link transmission delay. A RACH preamble transmitted in thereverse direction suffers an identical transmission delay. Due to thepropagation delay, the perceived arrival time of a RACH preamble at abase station is:

${t = \frac{2({distance})}{C}},\;{{{where}\mspace{14mu} C} = {3.0 \times 10^{8}\mspace{14mu} m\text{/}{s.}}}$

Due to this inherent delay, the range ambiguity for a subscriber unitvaries according to distance. At 100 m, the effect is negligible. At 30km, the delay may approach the transmission time of 4 symbols. Table 1illustrates the effect of round trip propagation delay.

TABLE 1 Effect of range ambiguity round trip time range (km) (msec) chipvalue symbol interval 0 0 0 1 5 0.033 137 1 10 0.067 273 2 15 0.100 4102 20 0.133 546 3 25 0.167 683 3 30 0.200 819 4

The first column is the distance in km between a mobile subscriber unitand a given base station. The second column is the round trippropagation delay of the RF signal in milliseconds from the base stationto a subscriber unit and back. The third column shows the chip clockingposition of the matched filter at the base station with time 0referenced at the start of a transmitted frame boundary. The valuerepresents when a first chip is received from a subscriber unitreferencing the beginning of a frame boundary. The fourth column showsthe expected location of the first output of the matched filter whichoccurs after assembling 256 received chips; (reference being made at thestart of a frame boundary). A symbol may be output during any one of thefirst four symbol intervals depending on subscriber unit distance.

Since the base station is not synchronized with the subscriber unit anddoes not have a carrier reference, the base station does not know wherein a received chip sequence the beginning of a RACH preamble symbolbegins. The matched filter must correlate a total of 256 chipscorresponding to a valid symbol pulse shape. As one skilled in this artknows, as the chips are received, the matched filter assembles 256 chipsto produce a first output representative of the pulse shape. Consecutiveoutputs from the matched filter are generated for each subsequentlyreceived chip.

The mobile subscriber unit transmits the preamble part first to accessthe RACH from the base station. One from among sixteen signatures israndomly selected and one from among five time-offsets is randomlychosen to account for the range ambiguity during transmission. Themobile subscriber unit constantly receives a frame boundary broadcastfrom the base station. To request a RACH, the mobile subscriber unittransmits a random burst with an n×2 ms time-offset (where n=0,1, . . .4) relative to the received frame boundary as shown in FIG. 3. Thetime-offset (value of n) is chosen at random at each random accessattempt.

Four received preamble signatures, a, b, c, and d are shown in FIGS. 4a–d received at the base station. Each signature arrives at one symbolduration (0.0625 ms) later due to round trip delay, with each signaturerepresenting a different distance between the base station and mobilesubscriber unit. Only sixteen consecutive symbols have signalcomponents, the other matched filter outputs represent noise. It isknown that range ambiguity will destroy the orthogonality amongsignatures and degrade performance. The possibility exists that the basestation receiver could confuse any combination from a possible nineteenoutputs from the matched filter as an incorrect signature.

Accordingly, there exists a need for a CDMA transmission and detectionscheme that is accurate notwithstanding communication distance and theeffects of Doppler.

SUMMARY

A preamble for a random access channel (RACH) transmission is providedby selecting one out of sixteen preamble signatures. Each of the sixteenpreamble signatures having sixteen symbols. A code is produced based onthe preamble sequence. The produced code is phase rotated to produce apreamble code.

BRIEF DESCRIPTION OF THE DRAWING(S)

FIG. 1 is a simplified block diagram of a prior art CDMA communicationsystem.

FIG. 2 is a table of sixteen coherent RACH signatures.

FIG. 3 is a timing diagram showing transmission timing for parallel RACHattempts.

FIG. 4A is a timing diagram showing a 16 symbol RACH preamble signaturereceived during the first symbol interval period.

FIG. 4B is a timing diagram showing a 16 symbol RACH preamble signaturereceived during the second symbol interval period.

FIG. 4C is a timing diagram showing a 16 symbol RACH preamble signaturereceived during the third symbol interval period.

FIG. 4D is a timing diagram showing a 16 symbol RACH preamble signaturereceived during the fourth symbol interval period.

FIG. 5 is a detailed block diagram of a CDMA communication system.

FIG. 6A is a prior art system diagram of a random access channelpreamble detector.

FIG. 6B is a random access channel preamble detector made in accordancewith the present invention.

FIG. 7A is a diagram of the symbol memory matrix.

FIG. 7B is a flow diagram of the procedure for tentatively detectingpreamble signatures.

FIG. 7C is a flow diagram of the procedure for resolving rangeambiguity.

FIG. 8 is a table showing four possible combinations of receivedpreamble signature symbols to resolve range ambiguity.

FIG. 9 is a table showing the relationship between orthogonality andrange ambiguity.

FIG. 10 is a table of sixteen non-coherent RACH signatures.

FIG. 11 is a system diagram of a non-coherent RACH preamble detector.

FIG. 12A is a system diagram of a coherent RACH preamble detectorcorrecting for multiple Doppler channels.

FIG. 12B is a detailed diagram of a preamble correlator.

FIG. 13 is an alternative embodiment of the present invention.

FIG. 14 is the encoding rule for the alternative embodiment of thepresent invention.

FIG. 15 is an uncoded sequence and its transformation into adifferentially coded sequence.

FIG. 16 is a transmitted signature of the sequences of FIG. 15.

DETAILED DESCRIPTION OF THE PREFERRED EMBODIMENT(S)

The preferred embodiment will be described with reference to the drawingfigures where like numerals represent like elements throughout.

A CDMA communication system 25 as shown in FIG. 5 includes a transmitter27 and a receiver 29, which may reside in either a base station or amobile subscriber unit. The transmitter 27 includes a signal processor31 which encodes voice and nonvoice signals 33 into data at variousrates, e.g. 8 kbps, 16 kbps, 32 kbps, 64 kbps or other rates as desiredfor the particular application. The signal processor 31 selects a ratein dependence upon the type of signal, service or in response to a setdata rate.

By way of background, two steps are involved in the generation of atransmitted signal in a multiple access environment. First, the inputdata 33 which can be considered a bi-phase modulated signal is encodedusing a forward error-correction (FEC) encoder 35. For example, if a R=½convolution code is used, the single bi-phase modulated data signalbecomes bivariate or two bi-phase modulated signals. One signal isdesignated the in-phase channel I 41 a. The other signal is designatedthe quadrature channel Q 41 b. A complex number is in the form a+bj,where a and b are real numbers and j²=−1. Bi-phase modulated I and Qsignals are usually referred to as QPSK.

In the second step, the two bi-phase modulated data or symbols 41 a, 41b are spread with a complex pseudo-noise (pn) sequence 43 a, 43 b. TheQPSK symbol stream 41 a, 41 b is multiplied by a unique complex pnsequence 43 a, 43 b. Both the I and Q pn sequences 43 a, 43 b arecomprised of a bit stream generated at a much higher rate, typically 100to 200 times the symbol rate. The complex pn sequence 43 a, 43 b ismixed at mixers 42 a, 42 b with the complex-symbol bit stream 41 a, 41 bto produce the digital spread signal 45 a, 45 b. The components of thespread signal 45 a, 45 b are known as chips having a much smallerduration. The resulting I 45 a and Q 45 b spread signals are upconvertedto radio frequency by mixers 46 a, 46 b, and are combined at thecombiner 53 with other spread signals (channels) having differentspreading codes, mixed with a carrier signal 51 to upconvert the signalto RF, and radiated by antenna 54 as a transmitted broadcast signal 55.The transmission 55 may contain a plurality of individual channelshaving different data rates.

The receiver 29 includes a demodulator 57 a, 57 b which downconverts thereceived revision of transmitted broadband signal 55 at antenna 56 intoan intermediate carrier frequency 59 a, 59 b. A second down conversionat the mixers, not pictured, reduces the signal to baseband. The QPSKsignal is then filtered by the filters 61 and mixed at mixers 62 a, 62 bwith the locally generated complex pn sequence 43 a, 43 b which matchesthe conjugate of the transmitted complex code. Only the originalwaveforms which were spread by the same code at the transmitter 27 willbe effectively despread. All other received signals will appear as noiseto the receiver 29. The data 65 a, 65 b is then passed to a signalprocessor 67 where FEC decoding is performed on the convolutionallyencoded data.

After the signal has been received and demodulated, the baseband signalis at the chip level. Both the I and Q components of the signal aredespread using the conjugate of the pn sequence used during spreading,returning the signal to the symbol level.

To establish a reverse-link from a mobile subscriber unit to a basestation, the mobile subscriber unit transmits a random access packettransported on a RACH. The transmission of the RACH is similar to whatwas described except the RACH does not undergo FEC. There may also bemore than one RACH employed in the communication system 25.

A table showing 16 possible coherent PSK coded RACH 71 preamblesignatures 73 is shown in FIG. 2. Each signature comprises 16 symbols.Each symbol A is a complex number A=1+j. A discussion of the codingmethods and complex numbers is beyond the scope of this disclosure andis known to those skilled in this art.

A prior art coherent RACH 71 detector 75 is shown in FIG. 6A. After thereceiver 29 demodulates the RACH 71 carrier, the demodulated signal 77is input to a matched filter 79 for despreading the RACH preamble 73.The output of the matched filter 79 is coupled to a preamble correlator81 for correlating the RACH preamble 73 with a known preamble pnsequence representing the preamble code 83. The output of the preamblecorrelator 81 will have peaks 85 corresponding to the timing 87 of anyreceived random access burst using the specific preamble code 83. Theestimated timing 87 can then be used in an ordinary RAKE 89 combiner forthe reception of the data part of the RACH 71 burst. Although thisdetector 75 may work well under ideal conditions with the coherent PSKcoded preamble signatures 73 shown in FIG. 2, its operation may beadversely affected due to range ambiguity and the presence of Doppler.

In a first embodiment of the present invention, non-coherent detectioncan be utilized. In this embodiment, the coherent RACH preamblesignatures 73 shown in FIG. 2 are differentially encoded, (i.e.,differential phase shift keyed (DPSK) processed). Accordingly, thecoherent preamble signatures 73 are first translated into incoherent,DPSK coded signals prior to transmission, and then differentiallydecoded after reception.

The method of translating the coherent symbols into non-coherent symbolsis performed in accordance with the following steps, (where i=rowsandj=columns). First:if S _(old)(i,1)=−A; multiply all j corresponding to i by−1.  Equation 2For example, for signature 4 (i=4) shown in FIG. 2

1 2 3 4 5 6 7 8 9 10 11 12 13 14 15 16 4 −A A −A A −A −A −A −A −A A −A A−A A A A multiply by −1 4 A −A A −A A A A A A −A A −A A −A −A −AAfter the first step, the old preamble signatures would consist of theoriginal undisturbed signatures (1, 3, 5, 8, 9, 11, 12 and 13) and thesignatures multiplied by −1 (2, 4, 6, 7, 10, 14, 15 and 16).

The second step of the translation process translates each consecutivesymbol of a preamble signature 73S _(new)(i,j)=A if: S _(old)(i,j)=S _(new) (i,j−1)  Equation 3S _(new)(i,j)=−A if: S _(old)(i,j)≠S _(new) (i,j−1)  Equation 4Continuing with the example, for signature 4 (i=4):S _(old) (4,2)≠S _(new) (4, 2−1)−A≠Atherefore;S _(new) (4, 2)=−A

The remainder of the DPSK coding is performed for each consecutivesymbol of a given preamble signature 73. The process translates all 16preamble signatures 73 into the differential preamble signatures 97shown in FIG. 10. The DPSK translation may be calculated and loaded intofirmware as part of the mobile subscriber unit or may be calculated wheninitiating a call depending upon the sophistication of the base stationreceiver. For the DPSK preamble signatures, the same process ashereinbefore described for coherent processing may performed except thatthe received signal must be recovered by differential decoding beforecorrelating with the preamble signatures.

A RACH detector 101 made in accordance with present invention 95 isshown in FIG. 6B. As previously described with reference to the priorart receiver 75, the received RACH 77 is demodulated and coupled to theinput of the matched filter 79. The output of the matched filter 79 iscoupled to the RAKE 89, to a time delay 103 and a first mixer 105. Eachreceived signature 97 is delayed by one symbol length, which is 256chips. The output of the time delay 103 is coupled to conjugateprocessor 107 which converts the received symbol to its complexconjugate. The output of the complex conjugate processor 107 is coupledto the first mixer 105 where the real part of the complex number isselected 106 and multiplied by the signature symbol and output to thepreamble correlator 81. The preamble correlator 81 correlates a possiblesignature with a sequence of outputs. This sum is compared to athreshold in the peak detector 85 and if it exceeds the threshold by theend of the sixteenth symbol, it is determined that a signature has beendetected. Since there are 16 computations, one for each signature, theremay be more than one accumulation that exceeds its threshold for a givensample time. In this case, the accumulation with the largest value isselected as correct. The estimated timing 87 can then be used in anordinary RAKE 89 combiner for the reception of the data part of the RACH71 burst.

In accordance with a second embodiment of the present invention, theenergy from each output of the RACH detector matched filter 79 iscomputed. Although the matched filter 79 is typically sampled at thechipping rate, it may be oversampled at twice or four times the chippingrate, (or even higher). In this embodiment, the chipping rate is 4.096million chips per second, or one chip every 0.244 μs.

Shown in FIG. 7A is a memory matrix 101 stored in RAM 100 where thevalue of the energy computed for each symbol output from the matchedfilter 79 is stored. The matrix 101 is arranged to store all possibledelayed symbol values corresponding to base station to subscriber unittransmission distances ranging from 100 m to 30 km. The matrix 101consists of 256 rows (0–255) 102 and 19 columns (0–18) 104 representingthe total number of chips transmitted during a RACH preamble signature.If the subscriber unit were located adjacent to the base station wherepropagation delay would be negligible, the first symbol would be outputafter 256 chips were received or at P(255,0). If the subscriber unitwere located at 30 km, the first symbol would be output after 819 chipswere received or at approximately P(54,4). Regardless of transmissiondistance, every 256 chips later would produce another symbol, and so on,thereby completing an entire row. Since sixteen symbols define apreamble signature, the matrix 101 allows for three additional symboloutputs to anticipate range ambiguity, (shown in FIG. 4, as will beexplained in greater detail hereinafter). Once the matrix 101 ispopulated, it includes all samples of interest for the mobile subscriberunit out to a range of 30 km.

Each output 97 from the matched filter 79 is a complex number:z(ik)=x(ik)+jy(i,k); where i=0 to 255 and k=0 to 18.  Equation 5The value for instantaneous energy, which is the sum of the squares ofreal and imaginary parts of each output, is computed as:P(i,k)=z(i,k) z(i,k)*=x ² +y ²,  Equation 6and stored within the matrix 101.

Because a preamble signature is from a set of 16 symbols, each with apre-specified chip pattern, a match filter output is expected to producea larger than average output 16 times, each larger value separated fromthe previous one by 256 chips. The combined output is the sum of thematched filter outputs speed by 256 chips. One problem that must beovercome is that the first matched filter output does not automaticallyoccur within the first 256 chips. It can occur later, as shown in Table1, depending upon the distance between the mobile subscriber unit andthe base station.

When a preamble signature is present, its corresponding matched filteroutputs will fill 16 of the 19 elements of one of the 256 rows 102. Foreach row, a complete preamble signature may be detected where the valueof total energy summed for the row exceeds a predetermined threshold.

Referring to FIG. 7B, the procedure 200 for tentatively detectingpreamble signatures is shown. Once the matrix 101 is populated (step201), the value of energy for each row is summed 109 and similarlystored (step 202). For those rows where the sum exceeds a predeterminedthreshold, the row is considered to be a “tentative detection”. The sumfor the first row is compared to a predetermined threshold (step 204) todetermine if the sum exceeds the threshold (step 206). If so, the row ismarked as a tentative detection (step 208). If each row has not beensummed (step 210), the next row is retrieved (step 212) and the processis repeated (steps 206–210). Once all the rows have been summed, therange ambiguity on each tentative detection is resolved (step 214),(which will be described in greater detail hereinafter), and thecandidates are output (step 216).

As indicated above, due to the location of the mobile subscriber unit,range ambiguity is introduced whereby the preamble signature may notoccur for up to four symbols. This range ambiguity must be resolved.Accordingly, for each row marked as a tentative detection, the value ofthe energy of the 16 consecutive positions within that row which producethe highest sum must be determined. Due to range ambiguity, fourpossible cases 1, 2, 3 and 4 are derived from a received version of apreamble signature. The four cases are shown in FIG. 8. In this example,signature 1 was transmitted and assembled from nineteen receivedsymbols, forming one row of the memory matrix 101. For each case,sixteen consecutive symbols out of nineteen are correlated with each ofthe sixteen possible preamble signatures, resulting in 64 hypotheses.One of the 64 hypotheses will result in a signature having the greatestenergy received. The greatest of the 64 hypotheses will occur in case 1since case 1 has all consecutive symbols and does not include noise.Cases 2, 3 and 4 include symbols derived from noise components and willnot correlate with one of the sixteen preamble signatures.

Referring to FIG. 7C, the procedure 300 for resolving range ambiguity inaccordance with the present invention is shown. As was described withreference to FIG. 8, each row comprises 19 total positions. Referringback to FIG. 7C, the values of the energy of the first 16 consecutivepositions of a row considered to be a tentative detection are analyzed(step 301). The energy sum for the 16 positions is calculated (step 302)and then stored (step 304). If the sums of all positions within the rowhave not been calculated (step 306) the next 16 consecutive positions,corresponding to elements 2–17, are reviewed (step 308). The counter isthen incremented (step 310) and the procedure is then repeated (step302-306). Once the sums of all positions have been calculated, all ofthe sums are compared to determine if the 16 consecutive positionswithin the row that have the greatest sum (step 312). The system thenoutputs the value of the column (k) corresponding to the beginning ofthe 16 consecutive positions having the greatest sum (step 314). This isa selected candidate. This procedure is repeated for each tentativedetection.

The process described with reference to FIG. 7C can be summarized inpseudo code as follows:

-   -   row i(i=0 to 255)    -   sum(k)=0; k=0,1,2,3    -   for k=0 to 3, do    -   sum(k)=sum(k)+P(i,n+k−1)    -   next k        then;    -   Select k for max sum(k)    -   maxk=0    -   max=sum(0)    -   for k=1 to 3    -   if sum(k)>max then    -   max=sum(k)    -   maxk=k    -   next k

The selected candidates are compared with the output of a normalcorrelation detection process for coherent or incoherent PSK coding. Thediscussion of a normal correlation detection process is beyond the scopeof this application and is well known to those skilled in this art.

Referring to FIG. 9, a table of the relationship between orthogonalityand range ambiguity is shown. The first column is the signature withwhich a received signal correlates. The second though fifth columns arethe correlation values of cases 1–4. The larger the correlation value,the better the received match to the received signal. A zero correlationvalue indicates the received symbol is orthogonal to the respectivesignature symbol. As can clearly be seen, orthogonality does not existamong the respective signatures for cases 2, 3 and 4.

The correlation values shown in FIG. 9 are calculated as:

$\begin{matrix}{{{\frac{100}{1024}{{{\overset{\rightarrow}{s}}^{(1)} \cdot {\overset{\rightarrow}{s}}^{\overset{H}{(k)}}}}^{2}} = {\frac{100}{1024}{{\sum\limits_{i = 0}^{15}{P_{i}^{(1)} \cdot P_{i + l}^{*{(k)}}}}}^{2}}},{k = 1},2,{\ldots\mspace{11mu} 16}} & {{Equation}\mspace{14mu} 7}\end{matrix}$where k=1 for signature 1, k=2 for signature 2, . . . k=16 for signature16 and for case 1, l=0; case 2, l=1; case 3, l=2; and case 4, l=3. Thevalue 1024 is derived by:

$\begin{matrix}{{1024 = {{{\overset{\rightarrow}{s}}^{(1)} \cdot {\overset{\rightarrow}{s}}^{\overset{H}{(1)}}}}^{2}},{{{where}\mspace{14mu}{\overset{\rightarrow}{s}}^{(1)}} = {{signature}\mspace{14mu} 1}},} & {{Equation}\mspace{14mu} 8}\end{matrix}$and where

$\begin{matrix}{{{{\overset{\rightarrow}{S}}^{(1)} \cdot {\overset{\rightarrow}{S}}^{\overset{H}{(1)}}} = {{\left\lbrack \underset{\underset{16\mspace{14mu}{symbols}}{︸}}{\begin{matrix}A & A & A & {- A} \\{- A} & {- A} & A & {- A} \\{- A} & A & A & {- A} \\A & {- A} & A & A\end{matrix}} \right\rbrack\;\begin{bmatrix}A^{*} \\A^{*} \\A^{*} \\{- A^{*}} \\{- A^{*}} \\{- A^{*}} \\A^{*} \\{- A^{*}} \\{- A^{*}} \\A^{*} \\A^{*} \\{- A^{*}} \\A^{*} \\{- A^{*}} \\A^{*} \\A^{*}\end{bmatrix}}\mspace{95mu} = {{16 \times {A \cdot A^{*}}}\mspace{95mu} = {{16 \times ({Hj})\left( {1 - j} \right)}\mspace{95mu} = {{16 \times 2}\mspace{95mu} = {{32\mspace{11mu}{{AND}\mspace{14mu} A}} = {{1 + {J\mspace{65mu} A^{*}}} = {{{conjugate}\mspace{14mu}{of}\mspace{14mu}{A\left( {1 - j} \right)}{therefore}\mspace{14mu} 32^{2}} = 1024}}}}}}}}} & {{Equation}\mspace{14mu} 9}\end{matrix}$

A RACH detector 95 made in accordance with this embodiment of thepresent invention is shown in FIG. 11. As previously described in theprior art receiver shown in FIG. 6, the received RACH 77 is demodulatedand coupled to the input of the matched filter 79. The output of thematched filter 79 is coupled to the RAKE 89, to a time delay unit 103, afirst mixer 105, and a first processor 99. Each received preamblesignature 97 is delayed by one symbol length T_(s), which is 256 chips,in the delay unit 103. The output of the time delay unit 103 is coupledto conjugate processor 107 which converts the received symbol to itscomplex conjugate. The output of the complex conjugate processor 107 iscoupled to the first mixer 105 where the real part of the complex numberis multiplied by the preamble signature symbol and output to thepreamble correlator 81. The preamble correlator 81 correlates a possiblesignature with a sequence of outputs based on the symbol sequence. Thissum is compared to a threshold, and if it exceeds the threshold by theend of the sixteenth symbol, a signature is detected. Since there are 16computations, one for each signature, there may be more than oneaccumulation that exceeds its threshold for a given sample time. In thatcase, the accumulation with the largest value is selected as correct.

Contemporaneous with the above-described signature correlation, thematched filter 79 output 97 is input to the first processor 99 whichcomputes the value of the energy for each symbol output. For each energyvalue computed, it is stored in the memory matrix 101. As previouslydescribed, after the energy values have been computed for a row of 19symbols, a second processor 109 computes the summed energy for thatgiven row which is then stored in a second memory 111. It should benoted that the memory matrix 101 and the second memory 111 may actuallycomprise a single RAM memory, instead of two separate components asshown. The energy exceeding a predetermined threshold is a tentativedetection. After an accumulation of 256 possible signatures comprising19 symbols has been accumulated in the second memory 111, a thirdprocessor 113 compares the 256 energy levels to normal signaturedetection on a one-to-one basis, thereby cross-verifying the results ofeach process to arrive at the correct signature sequence received.

To account for multiple Doppler channels, an alternative embodimentresolves the channels similar to the four case approach discussed above.To account for the Doppler channels, a phase rotation is introduced. Thephase rotation corrects and compensates the phase changes experienceddue to Doppler spreading. For coherent detection with m Dopplerchannels, m×4×16 hypotheses are created. The greatest of m×4×16hypotheses is selected and the corresponding signature is identified.

If a received sequence is r(t), each time 19 samples r(nΔt), n=1,2,3, .. . 19, are collected, four cases, n=1,2,3, . . . 16 (case 1), n=2, 3,4, . . . 17 (case 2), n=3,4,5, . . . 18 (case 3), and n=4,5,6, . . . 19(case 4) are considered. To resolve Doppler, each case is thencorrelated with 16 signatures with m different phase rotationscorresponding to m Doppler channels. The outputs of the correlation withphase rotations are;

$\begin{matrix}{{y_{ik} = {\sum\limits_{n = 1}^{16}{{{r\left( {n\;\Delta\; t} \right)} \times {\overset{\rightarrow}{s}}_{i} \times {\exp\left( {{{- j} \cdot 2}\;\pi\; f_{0k}n\;\Delta\; t} \right)}}}^{2}}},} & {{Equation}\mspace{14mu} 10}\end{matrix}$where i=1, 2, 3, . . . 16; k=1, 2, 3, . . . m; 2Πf_(0k) is the phaserotation of kth Doppler channel; and s_(i), where i=1, 2, 3, . . . 16are possible signatures.

An example frequency rotation of five Doppler channels is: (f₀₁, f₀₂,f₀₃, f₀₄, f₀₅)=(−200 Hz, −100 Hz, 0, 100 Hz, 200 Hz); with a spacing of100 Hz in between. Each case generates m×16 hypotheses. Four casescreate m×16×4 hypotheses. The preamble signature with the greatestcorrespondence to m×16×4 hypotheses is selected.

A receiver using coherent detection with multiple Doppler channels madein accordance with this embodiment of the present invention is shown inFIGS. 12A–B. In FIG. 12A, the received RACH 77 is coupled to the matchedfilter 79 to correlate with a spreading code (256 chips). As discussedabove, one symbol is output from the matched filter every 256 chipsuntil nineteen symbol outputs are collected and stored in the memorymatrix 101. Sixteen consecutive symbol outputs among nineteen symboloutputs are assembled and the four cases are formed.

Each of the four sixteen consecutive sample cases is correlated in thepreamble correlator 119 with each of the sixteen preamble sequences on mDoppler channels. The generated m×16×4 hypotheses with are then storedin a second memory 121. The case with the greatest energy from them×16×4 hypotheses is selected 123 and the corresponding preamblesignature is identified. FIG. 12B shows a detailed block diagram of thepreamble correlator for a given preamble sequence and a given Dopplerchannel, (i.e., having frequency shift of f_(0k), k=1 . . . m).

An alternative embodiment of the present invention is based on the 16×16signature matrix shown in FIG. 13. In utilizing this embodiment of thepresent invention, a new signature set is created by differentiallyencoding the signature matrix set forth in FIG. 13. The encoding rule isas follows. First, S(i,k), M(i,k) and R(i,k) are defined as:S(i,k)=kth element of signature i;M(i,k)=kth element of proposed new transmitted signature set; andR(i,k)=kth element of the proposed new replica set, to be stored in thereceiver.

Then the elements are mapped as follows: map A→1 and B→j=sqrt(−1), andset M(i,0)=A=1 and set R(i,0)=A=1. For k=1 to 15 we have the following:M(i,k)=M(i,k−l)×S(i,k)  Equation 11R(i,k)=S*(i,k)  Equation 12*denotes complex conjugate:If S(i,k)=1, R(i,k)=1If S(i,k)=j, R(i,k)=−j

This rule can be summarized in FIG. 14, where the left column representsthe four possible values of M(i,k−1) and the top row represents the fourpossible values of S(i,k). FIG. 15 shows an original uncoded sequenceand its transformation into a differentially encoded sequence.

In the receiver, these symbols are the differentially decoded.Arbitrarily starting with D(0)=1, the decoded symbols D(k), k=0 . . . 15are given in terms of the received coded symbols C(k) as:D(i,k)=C(i,k)×C(i,k−1)*  Equation 13

The correlation against the preamble signature is then performed,whereby Sum (i)=0. For i=0 to 15Sum(i)=Sum(i)+D(i,k)_(C) R(,k)  Equation 14

The full new transmitted signature set is shown in FIG. 16. This sametechnique can be applied to the preamble signatures shown in FIG. 13 byreplacing A by B; and B by A.

1. A subscriber unit using a code associated with a preamble of a randomaccess channel (RACH) transmission, the subscriber unit comprising:means for selecting one out of sixteen preamble signatures; means forproducing a code based on the preamble sequence; and means for phaserotating the produced code to produce a preamble code.
 2. The subscriberunit of claim 1 wherein the produced code is used for correlation with areceived sequence.
 3. The subscriber unit of claim 1 wherein theproduced code is used to resolve Doppler for a received RACHtransmission.
 4. A subscriber unit using a code associated with apreamble of random access channel (RACH) transmission, the subscriberunit configured to produce the preamble code derived by selecting oneout of sixteen preamble signatures; the subscriber unit configured toproduce a code based on the preamble sequence; and the subscriber unitconfigured to phase rotate the produced code to produce a preamble code.5. The subscriber unit of claim 4 wherein the produced code is used forcorrelation with a received sequence.
 6. The subscriber unit of claim 4wherein the produced code is used to resolve Doppler for a received RACHtransmission.